Electron paramagnetic resonance, electron spin echo modulation and electron nuclear double resonance studies on the photoionization of N-alkylphenothiazines in cetyltrimethylammonium bromide–alcohol reverse micelles. Effects of alkyl chain length of alkylphenothiazines, reverse micellar water pool size and cosurfactant alcohol

Abstract

Neutral N-alkylphenothiazines (PC_n, n= 1, 3, 6, 9, 12, 16) are solubilized and photoionized in cetyltrimethylammonium bromide (CTAB)–alcohol reverse micelles at 77 K to investigate the optimum photoionization conditions by varying the cosurfactant alcohol, reverse micellar water pool size and pendant alkyl chain length. The photo-produced radicals are identified and quantified by electron paramagnetic resonance (EPR). The relative locations from deuteriated water at the interface are monitored by deuteron electron spin echo modulation (ESEM) and proton matrix electron nuclear double resonance (ENDOR).

The photoyields and deuteron modulation depths of PC_n compounds show a U-shaped trend with increasing pendant alkyl chain length which indicates U-shaped conformations for the longer alkyl chains. The photoyields of the PC_n compounds decrease with the water to surfactant mole ratio and correlate with decreasing deuteron modulation depths which indicate an increasing distance from the phenothiazine moiety to deuteriated interface water. The deeper penetration is attributed to a decreased packing density of the interface region of the reverse micelles. This is supported by decreasing ENDOR linewidths with increasing water pool size.

A change of cosurfactant alcohol from butanol to octanol slightly decreases the photoyield and the deuteron modulation depths. This reflects deeper solubilization and a decreased rate of photoinduced radical conversion.